Various processes used in the oil industry, such as thermal cracking, coking, visbreaking, catalytic cracking and steam cracking, produce large quantities of light olefinic hydrocarbon cuts which also contain acetylenic and diolefinic hydrocarbons.
The subsequent uses of these olefsins in the manufacture of polymers or fuel, for example, require selective elimination of the acetylenic and diolefinic hydrocarbons which is as complete as possible.
The usual technique for such purification is selective hydrogenation carried out in a stream of hydrogen in a reactor comprising a fixed bed of a catalyst constituted by a metal from group VIII. The feed, which arrives at ambient temperature, is generally pumped and reheated and reintroduced into the hydrogenation reactor. The hydrogen required is added in any suitable fashion. At the reactor outlet, the excess hydrogen and the gaseous hydrocarbons contained in the supply gas are removed from the liquid hydrocarbon-containing gas in any suitable fashion, generally in a separator drum followed, if necessary, by a topping column.
European patent application EP-A 1-0 556 025 recently described a process for selective hydrogenation of diolefins in a light olefinic hydrocarbon refinery cut. In this technology, hydrogenation of the diolefin is internal of the rectification zone of the distillation column, the catalyst itself acting as a distillation structure. This mode of operation economises on equipment. However in this type of technology, where the reaction and distillation proceed simultaneously in the same physical space, the liquid phase descends through the whole catalytic bed in the reaction zone in rivulets or streams of liquid. The gas phase containing the vaporised fraction of the feed and hydrogen rises through the catalytic bed in columns of gas. Since the hydrogen necessary for reaction is in the gas phase, hydrogenation occurs essentially in that phase on the portion of the catalyst in contact with the fraction of hydrocarbons which is in the gas phase.
In that technology, then, the lightest hydrocarbons are preferentially hydrogenated. Thus in the cited patent application, the hydrocarbons containing five carbon atoms contained in the gasoline, which also comprises hydrocarbons containing at least 6 carbon atoms, are hydrogenated. The hydrogenation reaction is carried out in the rectification zone of the depentaniser. Thus the diolefins present in the C.sub.5 cut which go into the gas phase are preferentially hydrogenated.
Such a technology cannot be put into practice on a general basis and in some cases has such disadvantages as to render it impracticable. Thus, for example, in the catalytic cracking process intended to produce gasoline from heavy petroleum fractions, the production of a large quantity of essentially olefinic gaseous hydrocarbons which also contain small percentages of diolefins cannot be avoided.
In the most general mode for separating catalytic cracking products, "primary" fractionation is first carried out to separate the products from the reaction section for cracking light gases into liquefiable gases mixed with gasoline and into products which are heavier than gasoline. The core fraction, which generally contains hydrocarbons containing at least three carbon atoms to hydrocarbons containing ten carbon atoms, is then sent to a debutaniser which separates the gasoline at the bottom and the mixture of hydrocarbons containing three and four carbon atoms which is generally very rich in propylene and butenes and contains small quantities of diolefins, mainly butadiene. The mixture also contains numerous impurities, such as carbonyl sulphide (COS) and methylthiol (CH.sub.3 SH), and very often traces of arsine (AsH.sub.3). Such impurities strongly deactivate the hydrogenation catalyst when the latter operates on the above mixture and thus an initial purification treatment must be carried out to eliminate arsine and COS. As a result, the process described in European patent application EP-A1-0 556 025, which consists of placing the catalyst in a rectification zone which sees the simultaneous passage of the descending liquid phase and the ascending gas phase, cannot be carried out as the catalyst will be poisoned by AsH.sub.3 and COS present mainly in the gas phase, their boiling points being respectively -55.degree. C. and -50.degree. C.